While the ammonia and Diet Coke fountains illustrate chemical and physical laws, they're not that useful technologically. There is one fountain, the atomic fountain, that forms the basis of extremely accurate clocks. If we force atoms to flow upwards in a fountain, they will eventually fall downwards under the influence of gravity. As the atoms fall, they are weightless, and this reduces the uncertainty in a particular measurement.

Accuracy in the frequency measurement is enhanced when the atoms in the cloud are motionless. At the peak of a fountain, the movement of atoms is small, so an atomic fountain allows creation of a highly accurate atomic clock. One such clock is the NIST-F1 at the US National Bureau of Standards and Technology (see photo). This clock, active since 1999, measures a hyperfine transition in a fountain of cesium-133 atoms. The second is defined as 9,192,631,770 periods of this transition.

In the molecular fountain experiments, the applied electric fields were able to decelerate the molecules to speeds between 1.4 and 1.9 meters per second. After they peaked in the fountain, they were observed as they fall back under gravity.[4-5] The corresponding transverse temperature of the molecules at that time was less than, or equal to, 10 μK, and the longitudinal temperature was less than, or equal to, 1 μK.[4-5]

The molecular free-fall period was up to 266 milliseconds, which allowed sub-hertz frequency mesurements.[4-5] This measurement time is more than a hundred times larger than that provided by other techniques.[6] The molecules were ionized using a laser beam and detected to differentiate them from molecules that are present in the background gas.[6] At this time, just a single molecule can be examined every five fountain launches, so less than a molecule per second can be examined.[6] Future improvements are expected.[6]

References:

You can also tell my age by my use of "lighted," and not "lit," as the past tense of light, an explanation of which can be found at the Grammar Girl Web Site.